The present invention relates in general to current transformers and, more particularly, to current sensing devices for providing accurate current to meters and protection devices.
Intelligent Electronic Devices (IEDs) are well known. IEDs include, for example, electronic trip units, protective relays, energy meters and power quality meters. By way of example, a protective relay typically is connected to the secondary side of one or more current sensors coupled to a power line. The current sensors provide analog signals indicative of the power line signals to the protective relay. In the case of IEDs, the analog signals are converted by an analog to digital (A/D) converter to digital signals which are processed by a microcontroller. Alternatively, where older equipment is in use, the analog signal is connected to an analog protective device, such as an electromechanical relay or analog meter. In either case, an analog signal of appropriate magnitude and reflecting a proportional signal to a current on a power line is needed for input to the downstream protective device.
One type of current sensor, a current transformer (CT), is designed to provide a current in its secondary winding which is proportional to the current flowing in its primary winding. Current transformers (CTs) are commonly used in metering and protective relaying in the electrical power industry, including mid-voltage industrial applications, where they facilitate the measurement of large currents, often in the presence of high voltages. The current transformer isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.
Current transformers of the industrial scale are typically constructed by passing a single primary turn (either an insulated cable or an uninsulated busbar) through a well-insulated toroidal core wrapped with many turns of wire. The busbar acts as a primary winding and the wire wrapped around the toroidal core acts as the secondary winding. Current transformers are used extensively for measuring current and monitoring the operation of a power system. The current transformer is typically described by its current ratio from primary winding to secondary winding. Common secondary currents are 1 ampere (A) or 5A.
The current transformer's secondary current provides the general function of powering devices such as low voltage relays, IEDs or meters. While current transformer designs vary widely, each must address the requirements of fitting within a given volume of space, such as within a circuit breaker housing (i.e. mid-voltage (600V) industrial switchgear), and providing the desired level of accuracy when sensing the and stepping up current.
Presently, most critical current sensing solutions are provided with large, heavy current transformers. Electronic and fiber sensing options have been available, but not adopted, due to the loss of system reliability from associated additional components. Solutions are not available to interface with existing 5A or 1A IEDs or the like, and compensate for transmission line losses, saturations losses, etc. and line losses caused by the secondary current traveling a long distance to the IED. Input current for IEDs is traditionally 5A but could be, for example, 1A if the IED is specified for 1A input.
While CT designs vary widely, each must address the requirements of fitting within a given volume of space, such as within a circuit breaker housing, and providing the desired level of accuracy when sensing the circuit current. A predetermined maximum core volume is required within the current transformer to ensure that the current transformer does not become magnetically saturated upon the occurrence of overcurrent conditions when used within compact circuit breakers having variable ampere ratings. Alternatively, a predetermined minimum core volume is required to insure that the core will become sufficiently magnetized at the lower steady-state operating current levels.
With regard to limiting CT size, a single iron core current transformer has been used to both sense the circuit current along with providing operational power to the electronic trip unit in higher ampere-rated circuit breakers. To prevent the iron cores from becoming saturated at higher current levels, expensive magnetic steel laminates have been used and the core size increased to allow for overload and short circuit current sensing.
With regard to circuit current sensing, iron core current transformer for providing trip unit operating power and air core current transformer for circuit current sensing have been used. However, the use of two current transformers in each pole of a circuit breaker is not always feasible because of volumetric constraints. While an improved packaging arrangement of a combination iron and air core current transformers are available, the resultant specialized winding and assembly techniques result in a higher cost design. Such an arrangement is still subject to the saturation considerations when high-currents are involved or when volumetric constraints limit the amount of ferromagnetic core material that can be used.
Magnetic cores are employed in conventional circuit breakers, double break rotary circuit breakers, residential circuit breakers, commercial circuit breakers, industrial circuit breakers, air circuit breakers, overload relays, power meters, or any similar device providing electric circuit protection. Applications involving magnetic cores in circuit protective devices include, but are not limited to, the utility, industrial, and commercial industries. An illustration of prior art is shown in FIG. 1, where a CT having a primary winding comprising mid-voltage industrial busbar and a secondary winding providing secondary current output to an IED (such as a protective relay) located a great distance from the CT for use as operating current and trip current. The distance between the CT secondary winding and the IED factors into greater line losses which are a function of I2R. A single phase circuit is shown (for ease of illustration) whereas typically electric power distributions systems, such as mid-level industrial systems, are three phase and operating at a voltage of, for example 600V. The current transformer 97 of FIG. 1 is large, requiring more area in a switchgear frame and is rated for step-down current large enough in magnitude to operate the IED connected down line (typically rated 5A). The greater step-down current requires more windings than a current transformer with relatively less step-down current and hence the current transformer secondary has more windings than a current transformer with a larger secondary current; hence the current transformer heavier and occupies a greater volume. For example, the prior art CT, such as a Model 785 current transformer manufactured by Instrument Transformers Inc., a division of GE Mutlin, a subsidiary of the assignee of the present invention, weighs 58 lbs. for a single CT or 174 lbs. for three CTs (three-phases). The rating, size, and weight can be determined by one of ordinary skill in the art.
Therefore, based at least on the foregoing summarized discussion, a need exists for a current sensing device that reduces the need to compensate for losses. This novel, current sensing device includes several unique capabilities, including, as non-limiting examples: (1) the ability to provide current to a device without worrying about losses; (2) the ability to fit in smaller spaces; (3) the ability to be specified with lesser weight and hence decrease shipping cost; (4) the ability to sense current accurately; and (5) the ability to sense current and conserve energy. In one embodiment, the current sensing apparatus fits in a switchgear cabinet and is capable of providing current to a down stream device accurately, without the need for tedious, time consuming and often inaccurate loss calculations.